This self-contained essay collection is published to commemorate half a century of Bell’s theorem. Like its much acclaimed predecessor “Quantum [Un]Speakables: From Bell to Quantum Information” (published 2002), it comprises essays by many of the worlds leading quantum physicists and philosophers. These revisit the foundations of quantum theory as well as elucidating the remarkable progress in quantum technologies achieved in the last couple of decades. Fundamental concepts such as entanglement, nonlocality and contextuality are described in an accessible manner and, alongside lively descriptions of the various theoretical and experimental approaches, the book also delivers interesting philosophical insights. The collection as a whole will serve as a broad introduction for students and newcomers as well as delighting the scientifically literate general reader.

Reinhold Bertlmann (Professor of Physics in Vienna) is known for his work on anomalies in quantum field theory and on gluon condensates and potential theory and for always wearing two socks of different color as an illustration of the properties of quantum entanglement, used by John Bell, with whom he then collaborated at CERN, as the red thread in his seminal paper “Bertlmann’s Socks and the Nature of Reality”.

Anton Zeilinger (Professor of Physics in Vienna) explores the foundations of quantum physics and their applications to quantum information science and technology. His experiments on the entanglement of photons, which Einstein called “spooky action at a distance”, have become central to the emerging field of quantum information science where Zeilinger performed pioneering realizations of quantum teleportation, quantum cryptography, and quantum computation.